There are known in the art techniques for intercepting targets by utilizing only PIR electro-optical seekers. A typical system that exploits solely passive means is described for example, in: "Ching Fang Ling; Modern Navigation Guidance and Control Processing, Volume II, Chapter 8.", published by Prentice Hall.
In a system of this kind, the interceptor is fitted with passive tracking means for detecting infrared radiation emitted from the target exhaust, or body.
The underlying concept of the tracking procedure is based on detecting the infrared (IR) radiation emitted from the target's plume and calculating therefrom the line of sight (LOS) towards the target.
After having obtained consecutive LOS measurements (and their derivative LOS rate measurements) the target's state is calculated. Target state, in the context of the invention, stands for at least target's position and velocity from among position, velocity and acceleration. Alternatively, in the context of the invention, target state stands for range and either or both of LOS and LOS rates measurements. For convenience, in the description below, whenever "LOS measurement" is referred to, it encompasses either or both of LOS measurements and LOS rates measurements, with respect to an inertial reference frame.
In the case under consideration, the target state in terms of a target's spatial position x (i.e. position (p), velocity (v) and acceleration (a) is calculated.
Due to the fact that no active tracking means are utilized, PIR tracking technique has an inherent advantage in that the interceptor is less vulnerable to detection and is less susceptible to counter measures activated by the target. However, and as is well known in the art, the PIR tracking is error prone to the extent that the resulting estimated spatial position of the target is not of sufficient accuracy. Due to the accumulated error in estimating the target's spatial position, the target e.g. TBM, may be missed by the interceptor which may result in leakage of the TBM towards friendly territory, with an inevitable dire consequence.
Thus, when considering a target that flies at a relatively high velocity (such as air-to-air missile, or tactical ballistic missile that has gained sufficient velocity after boost phase), then the utilization of solely PIR tracking means will not bring about estimation of the target's spatial position at the desired high accuracy.
One possible approach of enhancing the accuracy of the estimated target's spatial position is by integrating active tracking means such as known per se laser range finder (LRF).
Detailed discussion of laser finders can be found in e.g. "Clifton S. Fox; Active Electro-Optical Systems", "The Infrared and Electro-Optical Systems Handbook" Volume 6, Chapter 2, 1993, published by "Infrared Information Analysis Center", Environmental Research Institute of Michigan Ann Arbor, Mich. USA.
Generally speaking, LRF includes a laser transmitter capable of generating and transmitting a beam of radiation that strikes the target, and the reflected portion of said radiation that is scattered from the target, is partially collected by a receiver that forms part of the LRF. On the basis of the elapsed time between transmitted and received radiation, the range to the target may be determined with a relatively high degree of accuracy.
The determined range and the LOS measurements (acquired from the passive tracking means) enable to derive the spatial position of the target far more accurately, and accordingly the risk of missing the target is significantly reduced.
A laser range finder has however an inherent drawback in that in a so called terminal phase tracking, (and in particular in distances of more than about 3 km from the target,) the reflected radiation (i.e. "signal") and clutter (i.e. "noise") are not easily discerned, one with respect to the other, due to low signal to noise ratio.
Accordingly, the incorporation of accurate target range measurements as derived from the LRF, may be utilized only at a relatively late stage of the interception phase, which in some cases, may prohibit timely steering of the interceptor for successfully homing onto the target, bearing in mind the inherent slow operation of the steering mechanism and that at this late stage, both the interceptor and the target flying at very high relative velocities (referred to as "closing velocity"). Thus, by way of example, for intercepting TBM at high altitude in an head-on trajectory, a typical closing velocity may exceed 3000 m/s.
It is accordingly the object of the present invention to obtain preliminary detection of the target by utilizing LRF even under low signal to noise prevailing conditions, and to thereby enhance the likelihood of timely steering the interceptor so as to enhance the probability of successful homing onto the target.
It is a specific object of the present invention to obtain improved probability of homing at the target when a laser range finder of the kind specified is fitted in an interceptor missile designate to destroy tactical ballistic missiles that are launched towards a friendly territory.